The present invention relates to a light measuring apparatus for obtaining characteristics of light from a light source or light to be measured such as light reflected by or transmitted through an illuminated sample and particularly to a spectral color meter for measuring color luminance values and the like of visible light (monochromatic light) and a method for measuring monochromatic light.
A color meter, one mode of a light measuring apparatus, for measuring color values of an incident light needs to have spectral responsitivities approximate to color matching functions of a standard observer recommended by the CIE (International Commission on Illumination). In recent years, with an increase of light sources for emitting a monochromatic light or a combination of monochromatic lights such as liquid crystal displays, plasma displays or LEDs (semiconductor light emitting devices), there has been an increasing demand for a further improvement in approximation precision to the color matching functions in order to improve the measurement precision of color values of these light sources.
Systems for realizing the spectral responsitivities approximate to the color matching functions include (a) a system by a color luminance meter directly reading stimulus values and (b) a system by a spectral luminance meter.
(a) The color luminance meter directly reading stimulus values builds filter characteristics approximate to the color matching function by combining a plurality of optical filters. This system has an advantage of compacting the light measuring apparatus since an optical system is simple and it is sufficient to provide a small number of light receiving sensors. However, there is a disadvantage of being difficult to meet a required approximation precision due to the restriction of usable filters and variations in the characteristics of the usable filters.
(b) The spectral luminance meter is for measuring spectral data at minute wavelength intervals using a narrow half power band width, for example, for measuring the luminance or chromaticity of a light to be measured (measurement light) from a spectral data obtained at wavelength pitches of 1 nm using a half power band width of about 5 nm (i.e., a visible wavelength region is covered by about 400 light receiving sensors). With such a spectral luminance meter, the spectral responsitivities can be caused to precisely coincide with the color matching functions, enabling highly precise luminance and chromaticity measurements, since the spectral data is obtained at the minute wavelength intervals. However, a bright optical system having a high resolving power is necessary in order to obtain spectral data as above, thereby presenting a problem of necessitating the use of a light measuring apparatus having a relatively large size.
In view of the above problems, there has been known a light measuring apparatus adopting a so-called spectral fitting method for conducting a measurement after obtaining a composite spectral responsitivity approximate to a specific spectral responsitivity by multiplying spectral responsitivities obtained at relatively long wavelength intervals (about several tens nm) using a wide half power band width by specified weights set beforehand. Such a light measuring apparatus is disclosed, for example, in Japanese Unexamined Patent Publication No. 2002-13981. Such a light measuring apparatus adopting the spectral fitting method has advantages that:
(I) The approximation precision of the spectral responsitivities is higher than the color luminance meter directly reading the stimulus values (a),
(II) A light amount is larger and a signal-to-noise (S/N) ratio is higher since each light receiving channel lies in a wide wavelength region, and
(III) A silicon photodiode array having better linearity, S/N ratio and dynamic range due to a smaller number of light receiving channel can be used, and it is not necessary to use a CCD (charged-coupled device) sensor array or a like electric charge transferring type sensor array inferior in these points.
Although the light measuring apparatus adopting the spectral fitting method has various merits as described above, the inventors of the present invention found out that the approximation precision of the composite spectral responsitivities might decrease unless the half power band width and the wavelength pitch (interval between center wavelengths of adjacent light receiving sensors) of the respective light receiving sensors used were optimized. Particularly, the influence of the decreased approximation precision is large in the measurement of color luminance values of monochromatic lights emitted from the aforementioned various displays and LEDs, with the result that there have been cases where the characteristics of the monochromatic lights could not be obtained with high precision.